1. Field of the Invention
This invention relates to a method of correcting the density of a subtraction image obtained by a subtraction processing of radiation images, and an apparatus for carrying out the method. This invention particularly relates to a method of correcting the density of a subtraction image obtained by a digital subtraction processing of radiation images conducted by use of stimulable phosphor sheets so that the density range of the subtraction image becomes always the same, and an apparatus for carrying out the method.
2. Description of the Prior Art
Conventionally, a digital subtraction processing method is used for processing radiation images. In the method, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to a subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a difference signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained.
Basically, subtraction processing is classified into the so-called temporal (time difference) subtraction processing method and the so-called energy subtraction processing method. In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to radiations having energy distributions different from each other to obtain two radiation images respectively containing the images of a specific structure recorded on the basis of the intrinsic radiation energy absorption characteristics of the specific structure. Then, the image signals of the two radiation images are weighted appropriately when necessary, and subjected to subtraction to extract the image of the specific structure.
Since subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved methods by use of electronic technology. The processing technique is specifically called the digital subtraction processing method, or more commonly, digital radiography (abbreviated as "DR").
A novel digital subtraction processing method has been proposed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-163340. The method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to the radiation passing through the same object under different conditions to have radiation images of the object stored in the stimulable phosphor sheets, image information on the specific structure being different between the radiation images, (iii) detecting the radiation images by scanning with stimulating rays to obtain digital image signals, and (iv) conducting a digital subtraction processing by use of the digital image signals. The stimulable phosphor sheets comprise a stimulable phosphor which is able to store a part of the radiation energy when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, and then emits light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, as disclosed for example in U.S. Pat. No. 4,258,264. The stimulable phosphor sheets exhibit an extremely wide latitude of exposure and a markedly high resolving power. Therefore, when the digital subtraction processing is conducted by use of the radiation images stored in the stimulable phosphor sheets, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, regardless of the amount of radiation to which the stimulable phosphor sheets are exposed.
The stimulable phosphor sheets can also be used for the aforesaid energy subtraction processing. However, when difference signals obtained by the subtraction processing are directly used for reproducing images, the density range (or luminance range when the images are reproduced on a cathode ray tube) becomes different among the reproduced images. This is because image signals detected from two or more radiation images for the subtraction processing are caused to fluctuate by any fluctuation in sensitivity of the stimulable phosphor sheets, fluctuation in sensitivity of the image readout apparatus, fluctuation in radiation exposure dose at the radiation image recording step, or the like. The difference signal obtained by the subtraction processing is very weak. Therefore, when the image signals fluctuate as described above, the level of the fluctuation component in the difference signal obtained from the image signals becomes markedly higher than the level of the correct signal component, and the density range of the subtraction image reproduced by use of the difference signal fluctuates largely. As a result, the subtraction image often becomes unsuitable for viewing, particularly for diagnostic purposes, so that it is not always possible to carry out correct diagnosis, for example, by comparing many subtraction images.
In order to obtain radiation images for the energy subtraction processing conducted by use of the stimulable phosphor sheets, i.e. radiation images wherein the image information on a specific structure is different, radiation energy to which the stimulable phosphor sheets are exposed may be changed by changing the tube voltage of the radiation source. Alternatively, a filter for changing the energy distribution of the radiation may be inserted between the radiation source and the object, or any other known method may be used. Also, the radiation images may be recorded by a single radiation exposure operation by use of a stimulable phosphor sheet-filter stack or the like as described in Japanese Patent Application No. 57(1982)-193765. This application has a U.S. equivalent, application Ser. No. 883,429, filed July 15, 1986 and commonly assigned with the present application. Ser. No. 883,429 is a continuation of Ser. No. 548,486, filed Nov. 3, 1983, now abandoned.